1. Field of the invention
The present invention relates to a method for manufacturing a translucent LCD, and more particularly to a method for manufacturing an array substrate of a translucent LCD capable of providing low contact resistance in a transmissive region and high reflectivity properties in a reflective region while simplifying processes.
2. Description of the Prior Art
As generally known in the art, LCDs include two substrates having electrodes formed thereon and a liquid crystal layer interposed between them. By applying a voltage to the electrodes, liquid crystal molecules in the liquid crystal layer are rearranged. As a result, the amount of transmitted light is adjusted, and predetermined images are realized.
LCDs may be classified into transmissive LCDs using a light source, such as a backlight, to display images and reflective LCDs using natural light as the light source. The transmissive LCDs can realize bright images even in dark surroundings, because they use a backlight as the light source. However, much power is consumed by the backlight. In contrast, the reflective LCDs consume little power, because they use natural light from surroundings instead of the backlight. However, reflective LCDs cannot be used when the surroundings are dark.
In an attempt to solve these problems, a translucent LCD has been proposed, the pixel unit of which is divided into transmissive and reflective regions so that, when no external light source exists (e.g. indoor or dark place), its own internal light source is used to display images and, when there is enough light (e.g. outdoor place), incident light from the exterior is reflected to display images.
FIG. 1 is a sectional view showing an array substrate of a conventional translucent TN (twisted nematic)-mode LCD disclosed in Korean Patent Publication No. 2004-0070716. A method for manufacturing the array substrate will now be described with reference to the drawing.
As shown, a gate metal layer is deposited on a glass substrate 1, the pixel unit of which has been divided into a TFT (thin film transistor) region TFT, a reflective region R, and a transmissive region T. The gate metal layer is patterned to form a number of gate lines (not shown) on the interface of the pixel unit while being arranged in the transverse direction, including a gate electrode 2 positioned in the TFT region TFT. A gate insulation layer 3 is formed on the front surface of the substrate 1 to cover the gate lines, including the gate electrode 2.
A non-doped amorphous silicon layer and a doped amorphous silicon layer are successively formed on the gate insulation layer 3 and are patterned to form active patterns in the TFT region TFT. An organic insulation layer 6 is selectively formed on a part of the gate insulation layer 3 positioned in the reflective region R, but not those in the TFT and transmissive regions TFT and T, in such a manner that the organic insulation layer 6 has a number of protrusions on its surface. The organic insulation layer 6 is patterned and, as a result, a first opening 7 is formed in the transmissive region T while extending through the organic insulation layer 6.
After forming the active patterns and the organic insulation layer 6, source and drain metal layers are deposited on the resulting substrate and are patterned to form a number of data lines (not shown) at the interface of the pixel unit while being arranged in a longitudinal direction so that they are substantially perpendicular to the gate lines. Simultaneously, source and drain electrodes 8a and 8b are formed in the TFT region TFT. A part of the doped amorphous silicon layer between the source and drain electrodes 8a and 8b is etched to form an ohmic layer 5 and a channel layer 4, which is made of the non-doped amorphous silicon layer. As a result, a TFT 10 is constructed in the TFT region TFT. When the data lines are formed, a reflective electrode 11 is simultaneously formed on the organic insulation layer 6 in the reflective region R, for example, while being integral with the source electrode 8a of the TFT 10.
A protective layer 12 is formed on the resulting substrate to cover the TFT 10 and the reflective electrode 11. The protective layer 12 is etched to form a via hole 13 for exposing the source electrode 8a and remove a part of the protective layer 12 in the transmissive region T, so that a second opening 14 is formed. An ITO layer is deposited on the protective layer 12 and is patterned to form a pixel electrode 15, which contacts the source electrode 8a through the via hole 13. This completes an array substrate.
As mentioned above, conventional translucent LCDs use source and drain metal layers, which are made of metal having a high reflectivity, as reflective electrodes. For example, the source and drain electrodes are composed of a single layer, which is made of any one of Al, Al alloy, Ag, and Ag alloy. Alternatively, the source and drain electrodes are composed of a dual layer, which includes a lower layer made of any one of Cr, Ti, and MoW and an upper layer made of Al or Ag.
However, any metal belonging to Al series has high contact resistance with ITO, which constitutes the pixel electrodes, and does not emit light easily. Even when light is emitted, mura and luminance deteriorate. This degrades screen quality. In contrast, any metal belonging to Ag series has low contact resistance with the ITO, and the problems related to the use of Al-series metal can be avoided. However, Ag-series metal is expensive and is practically impossible to use, considering the production cost.
In summary, conventional translucent LCDs using source and drain metal layers as reflective electrodes cannot meet consumers' expectations, due to problems related to contact resistance with the ITO layer and increased cost.
In the case of conventional translucent LCDs, a total of 8-11 masks are necessary to manufacture their array substrate. Particularly, 5-8 masks are used to form a transmissive region, and at least 3 masks are used to form a reflective region through processes for forming a via on a resin layer, forming embossing, and forming a reflective electrode.
FIG. 2 is a sectional view showing an array substrate of a conventional translucent TN-mode LCD. The array substrate is manufactured by successively performing: a first mask process for forming a gate line and a common electrode line, including a gate electrode 2; a second mask process for forming an active pattern; a third mask process for forming a data line, including source and drain electrodes 8a and 8b; a fourth mask process for forming a via which exposes the source and drain electrodes 8a and 8b; a fifth mask process for forming a pixel electrode 15 of ITO; a sixth mask process for forming a via 13 on a resin layer 6; a seventh mask process for forming embossing 16 in a reflective region R; and an eighth mask process for forming a reflective electrode 11 in the reflective region R.
When the reflective region R is formed, at least 3 masks are additionally used to form the via 13 on the resin layer 6, form the embossing 16, and form the reflective electrode 11, compared with the case of forming a transmissive region T.
FIG. 3 is a sectional view showing an array substrate of a conventional translucent FFS (fringe field switching)-mode LCD. The array substrate is manufactured by successively performing: a first mask process for forming a gate line and a common electrode line 2a, including a gate electrode 2, as well as forming an embossing pattern 2b in a reflective region R; a second mask process for forming an active pattern; a third mask process for forming a data line, including source and drain electrodes 8a and 8b; a fourth mask process for forming a via 9 which exposes the common electrode line 2a; a fifth mask process for forming a plate-type counter electrode 17 in a transmissive region T; a sixth mask process for forming an reflective electrode 11 in the reflective region R; a seventh mask process for forming a via 13 which exposes the source and drain electrodes 8a and 8b; and an eighth mask process for forming a slit-type pixel electrode 15 in the transmissive and reflective regions T and R.
When the reflective region R is formed in the FFS-mode LCD, at least 2 additional masks are used to form the via 9 on the resin layer 6 to expose the common electrode line 2a and form the reflective electrode 11, as compared with the case of forming the transmissive region T. As a result, a total of at least 8 masks are used for the whole substrate.
As mentioned above, conventional translucent LCDs use at least 8 masks to manufacture their array substrate. In addition, each mask process includes a process for applying a photosensitive layer, a process for exposing the applied photosensitive layer to light using a mask, a process for developing the exposed photosensitive layer, and a thermal process (e.g. soft or hard baking). Consequently, conventional methods for manufacturing an array substrate of a translucent LCD, which require at least 8 masks, are very complicated and incur great costs.
In FIGS. 2 and 3, reference numeral 1 refers to a glass substrate, 3 is a gate insulation layer, 4 is a channel layer, 5 is an ohmic layer, 10 is a TFT, 11a is an Al metal layer, 11b is an Mo metal layer, 12 is a protective layer, and 14a is an opening.